In preparing acrylic acid or preparing acrylic esters from acrylic acid by either the esterification reaction or the transesterification reaction, a distillation operation for separating, concentrating or purifying is typically employed. It is known that acrylic acid and its esters have high polymerization tendency which becomes extremely high at elevated temperatures such as in the distillation step. Therefore, in the preparation of acrylic acid and its esters on a commercial scale, the prevention of polymerization during the distillation step is extremely important to the stable operation of the process.
When distilling acrylic acid or its esters in the separation, concentration or purification step, it is known that a polymer may be formed at various places, for example, in the reverse side of the trays, the inside of the bubble caps, the external surface of the downcomers and the recessed portions on the column wall which are not perpetually wetted with a liquid containing polymerization inhibitors, as well as the hardware (e.g., bolts, nuts), the packing and the gaskets for setting up the trays at which liquid tends to stagnate. The polymers thus formed do not readily dissolve in acrylic acid, acrylic esters, water or other organic solvents. Further, once the polymer forms inside the column, it becomes a nucleus of polymerization to cause a gradual accumulation of polymer which can block the inside of the column and render impossible the continuous distillation operation. Moreover, great difficulty is involved in removing this accumulated polymer.
Previously, as a measure to inhibit the polymerization of acrylic acid or esters thereof during their distillation, there was proposed in the specification of the German Laid-Open Pat. No. 2,027,655 a distillation column of a construction in which the reverse side of the trays and the inside wall of the column have been made readily wettable. Aside from this proposal, the method most widely used heretofore for inhibiting the polymerization of these compounds is that of adding a polymerization inhibitor to the distillation column.
Typical polymerization inhibitors are phenolic compounds, amine compounds, nitro compounds, quinone compounds and the inorganic salts.
Illustrative of the methods of using these polymerization inhibitors is U.S. Pat. No. 3,666,795 which uses the combination of hydroquinone-phenoloxygen and U.S. Pat. No. 3,674,651 which uses the combination of diphenylamine-benzoquinone .(or hydroquinonemonomethyl ether)-oxygen.
U.S. Pat. No. 4,663,480 discloses that manganese nitrite, i.e. Mn(NO.sub.2).sub.2, effectively inhibits undesired polymerization of ethylenically unsaturated monomers such as unsaturated hydrocarbons, hydrocarboxyl acids and hydrocarboxyl esters. It discloses a process for inhibiting polymerization of a polymerizable ethylenically unsaturated monomer selected from the group consisting of polymerizable ethylenically unsaturated hydrocarbons, polymerizable ethylenically unsaturated acids and polymerizable ethylenically unsaturated esters, which comprises admixing therewith manganese nitrite in an amount effective for inhibiting polymerization, e.g. thermal polymerization, of the monomer. It is further disclosed that, in general, such amount may be for example from about 0.001 to about 0.05 part by weight per 100 parts by weight of the monomer. A composition is also disclosed which is a mixture of the above ethylenically unsaturated monomer and Mn(NO.sub.2).sub.2 in an amount effective for inhibiting thermal polymerization of the monomer.
U.S. Pat. No. 4,021,310 discloses carrying out the distillation for separating or purifying the acrylic acid obtained by the vapor phase catalytic oxidation of propylene or acrolein or acrylic esters derived from the aforesaid acrylic acid, in the presence of
(A) at least one compound selected from the group consisting of hydroquinone, hydroquinonemonomethyl ether, cresols, phenols, t-butyl catechol, diphenylamine, phenothiazines, various other compounds described in the patent, and methylene blue; PA1 (B) at least one compound selected from the group consisting of copper dimethyldithiocarbamate, copper diethyldithiocarbamate, copper dibutyldithiocarbamate and copper salicylate; and PA1 (C) molecular oxygen.
The '310 patent discloses that some of the compounds of groups (A) and (B), above, are known as polymerization inhibitors of acrylic acid or its esters and that the simultaneous use of these compounds with molecular oxygen is also known. In contrast to these known methods, the '310 patent describes the simultaneous use of the aforesaid three components of groups (A), (B) and (C) in the distillation of acrylic acid or its esters, thereby gaining a synergistic effect.
Thus as noted, polymerizable ethylenically unsaturated monomers such as hydrocarbons, acids and esters undesirably polymerize at various stages of their manufacture, processing, handling, storage and use. A particularly troublesome problem is equipment fouling caused by polymerization in the purification stages of monomer production processes. Polymerization, such as thermal polymerization, during the purification of monomers results in the loss of monomeric material and in loss of production efficiency because the polymer often deposits in or on equipment in contact with the monomers and must be removed at frequent intervals.
As has been discussed above, a wide variety of substances have been proposed for inhibiting uncontrolled polymerization, e.g., thermal polymerization, of the above ethylenically unsaturated monomers. However, the heretofore proposed substances have not been entirely satisfactory. Accordingly, there is a substantial need in the art for improved compositions in which polymerization of such monomers is inhibited during transport and storage as well as during the distillation process for purifying or separating the acrylic monomer.